Calculating rates of reactions: The rate of a chemical reaction can be found by measuring the quantity of a reactant used or the quantity of product formed over time:

The idea

Rate tells you how quickly a reaction proceeds. Measure it by tracking a reactant disappearing or a product appearing. For example, add marble chips to dilute hydrochloric acid and measure the carbon dioxide volume in a gas syringe every 10 s. The volume increases quickly at first, then slows as acid is used up. Alternatively, measure the decreasing mass of the flask as gas escapes. Plot quantity on the y-axis and time on the x-axis. The average rate over a time interval is the change in quantity divided by the time taken. For Higher Tier only, the gradient of a tangent at any point gives the rate at that specific moment; a steeper gradient means a faster reaction. Always record quantity and time together to calculate rate.

What earns the marks

• Rate can be followed by measuring a reactant being used up, such as the loss in mass of a reaction mixture when a gas escapes.
• Rate can be followed by measuring a product formed, such as the volume of gas collected in a gas syringe or the mass of precipitate formed.
• The quantity measured must be recorded at regular time intervals so that a rate can be calculated.
• The average rate over a time interval is the change in quantity divided by the time taken for that change.
• A graph of quantity against time allows the rate at any specific time to be found from the gradient of the tangent (Higher Tier only).
• The units of rate depend on the quantity measured and the time unit, for example cm³/s for gas volume or g/s for mass loss.

Common mistakes and how to put them right

• Plotting time on the y-axis and quantity on the x-axis.

  Put it right: This makes the gradient the reciprocal of the rate. Always put quantity on the y-axis and time on the x-axis.

• Confusing the total quantity measured with the rate.

  Put it right: The total volume of gas collected is not the rate. Rate is the quantity changed per unit time, so divide by the time taken.

• Assuming the rate stays constant throughout.

  Put it right: Many reactions slow down as reactants are used up, so the gradient of a quantity-time graph decreases over time.

Exam tips

• When describing a method, state the quantity you will measure, the time intervals, and how you will calculate the rate from the results.
• If asked to compare rates, quote values with units and refer to the steepness of the graph.
• For a tangent (Higher Tier only), draw it at the required time, make it as long as possible, and read the coordinates from points on the tangent to calculate the gradient.